1. Field of the Invention
The present invention relates to a transmission method and a transmitter circuit for transmitting a high-frequency signal by wireless.
2. Related Art
In a modulating signal involving frequency modulation, in general, especially multi-level modulation such as the quadrature amplitude modulation (QAM), it requires a linear operation for a high-frequency power amplifier disposed in a transmitter circuit for transmitting electricity for an antenna. An operation class A or AB has therefore been utilized for the high-frequency power amplifier.
However, broadbandization of communication has initiated the use of a communication method by a multicarrier such as the Orthogonal Frequency Division Multiplex (OFDM), thereby leading the conventional class A or class AB high-frequency power amplifier not to capable of achieving high-efficiency. In other words, in the OFDM modulation, superposing of sub-carriers generates momentarily a large amount of power completely at random, thereby a ratio of average power to momentary peak power, i.e., a Peak to Average Power Ratio (PAPR), becomes larger. Thus, it is required to hold large direct-current (DC) power constantly for allowing the momentary peak power being amplified linearly. Since the DC power set as such is too large for amplifying the average power, the excess DC power is wasted as heat. It results in the power supply efficiency being substantially reduced.
Consequently, continuous usable time of, for example, a portable wireless application utilizing a battery as the power supply becomes shorter, causing a practical problem.
For the purpose of solving such problems, there is proposed a conventional Envelope Elimination and Restoration (EER) method, known as the Khan's method.
FIG. 18 is a block circuit diagram schematically illustrating the conventionally known EER method (see Patent Documents 1 and 2, for example). Firstly, functions of respective blocks will be described.
In this figure, in a modulating signal generated by a modulating signal generating circuit 1901, a phase component and an amplitude component thereof are separately detected by a detection circuit 1902. In particular, the modulating signal generated by the modulating signal generating circuit 1901 is a complex signal including, In-phase (in-phase signal) and Quadrature (orthogonal signal). This modulating signal is detected as the amplitude component √{square root over ( )}(I2+Q2) and the phase component tan−1 (Q/I), separately. Since they are processed in the form of a digital signal, they are digital/analog converted by a digital/analog converter (D/A converter) for a subsequent process in an analog processing block. The D/A converter is included in the detection circuit 1902, for example. The D/A converter converts an input bit signal to an analog signal by means of, for example, an offset binary code. The offset binary code is one of conversion codes for handling positive and negative signals, wherein a maximum positive value is a maximum positive value of a dynamic range of a D/A converter output shifted by 1 LSB.
Hence, the phase component output from the detection circuit 1902 passes through the D/A converter to be output to an orthogonal modulator 1905 corresponding to a frequency conversion means. The orthogonal modulator 1905 multiplies an IQ signal from the D/A converter by a SIN-wave and a COS-wave, having an angular velocity of a carrier frequency (ω), respectively, to up-convert the phase component. Subsequently, the phase component converted to a modulated wave is input into a high-frequency input terminal of a high-frequency power amplifier 1906 in the form of a high-frequency power.
Moreover, the amplitude component passes through the D/A converter to be input into an operational amplifier 1907, and, after being amplified, input into an emitter follower 1908 corresponding to a DC conversion means. Output amplitude of the emitter follower 1908 is determined by input amplitude of the operational amplifier 1907 and a feedback amount of the output amplitude fed back to a feedback terminal, i.e., an inverting terminal, of the operational amplifier 1907. The feedback amount is determined by, for example, a resistance value ratio of voltage dividing resistors R, r of a feedback circuit 1903 corresponding to a feedback means. That is, the feedback amount for the operational amplifier 1907 is determined by r/(r+R), wherein a gain thereof is converting 1+R/r when the feedback terminal of the operational amplifier 1907 as a non-inverting amplifier is connected to the feedback circuit 1903, as shown in FIG. 18.
Reasons for forming a feedback loop as shown in FIG. 18 are that the amplitude component (voltage) of the output from a DC converter 1908 is required to be the linearly-amplified output from the D/A converter, and that both linear and non-linear distortions in the output from the DC converter 1908 are required to be as minimum as possible. Here, the linear distortion in the output from the DC converter 1908 represents disturbance of a frequency response due to a group delay. The non-linear distortion represents disturbance of an amplitude response due to a non-linearity of conductance of an active element.
The feedback loop is therefore formed so as to feed back the output from the D/C converter 1908 to the feedback terminal of the operational amplifier 1907 to decrease an error between the input into the operational amplifier 1907 and the output to the DC converter 1908. The voltage output from the DC converter 1908 in this manner is input into a power supply (drain or collector terminal) of the high-frequency power amplifier 1906. Assuming that the high-frequency power amplifier 1906 is of a saturation type, the output voltage from the high-frequency power amplifier 1906 is proportional to the power source voltage. Hence, the phase component and the amplitude component are multiplied with each other at the output side of the high-frequency power amplifier 1906, thereby the signal expressed by the following formula is obtained.
                                                        I              2                        +                          Q              q                                      ⁢        exp        ⁢                                  ⁢                  j          (                                    ω              ⁢                                                          ⁢              t                        +                                          tan                                  -                  1                                            ⁢                              Q                I                                              )                                    [                  Formula          ⁢                                          ⁢          1                ]            Which is down-converted by a demodulator (carrier frequency: ω), thereby the modulated wave up-converted linearly of the modulating signal expressed by the following formula can be obtained.
                                                        I              2                        +                          Q              q                                      ⁢        exp        ⁢                                  ⁢                  j          (                                    tan                              -                1                                      ⁢                          Q              I                                )                                    [                  Formula          ⁢                                          ⁢          2                ]            
Next, the conception regarding the power efficiency will be described.
Here, the modulating signal (number of sub-carriers 52, modulation bandwidth 16.25 MHz), used in the IEEE 802.11a standard, is discussed as an illustrative example of the OFDM modulating signal. The maximum momentary power of the OFDM signal can be obtained when the maximum amplitudes of all the 52 sub-carriers overlap. Where Ak (standardized at the maximum value), Bk, and ω are the amplitude, the phase, and a carrier wave frequency, respectively, the power is proportional to the following formula:
                              {                                    ∑                              k                =                1                            52                        ⁢                                          A                k                            ⁢              exp              ⁢                                                          ⁢                              j                ⁡                                  (                                                            k                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                                          B                      k                                                        )                                                              }                2                            [                  Formula          ⁢                                          ⁢          3                ]            
Assuming that all the sub-carriers take the maximum values, then Ak=1, the foregoing formula can be expressed as follows.
                              {                                    ∑                              k                =                1                            52                        ⁢                          exp              ⁢                                                          ⁢                              j                ⁡                                  (                                                            k                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                                          B                      k                                                        )                                                              }                2                            [                  Formula          ⁢                                          ⁢          4                ]            
By expanding this formula, expressed is an addition of the sum of squares of the respective sub-carriers and a doubled sum of the product of the different sub-carriers. That is, the momentary peak power is obtained by adding up a sum of the power of the respective 52 sub-carriers and a doubled number of combinations to select 2 sub-carriers among the 52 sub-carriers as a contribution of the sum of the product of the different sub-carriers (since the amplitude equals to 1, the product equals to 1). The resulting expression is 52+2*52C2=52+52*51.
As for the average power, the following formula is integrated by a cycle of a fundamental wave 1/312.5 kHz.
                              {                                    ∑                              k                =                1                            52                        ⁢                                          A                k                            ⁢              exp              ⁢                                                          ⁢                              j                ⁡                                  (                                                            k                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                                          B                      k                                                        )                                                              }                2                            [                  Formula          ⁢                                          ⁢          5                ]            However, since all the sub-carriers are orthogonal, the contribution of a sum of the product of the different sub-carriers equals to 0. The average power is thus expressed by the following formula, which is the sum of the average power of the respective 52 sub-carriers. Here, the symbol “< >” represents a time average.
                              ∑                      k            =            1                    52                ⁢                              〈                          A              k                        〉                    2                                    [                  Formula          ⁢                                          ⁢          6                ]            
Since a primary modulation (BPSK, QPSK, 16 QAM, 64 QAM) of the OFDM modulation is not filtered, an amplitude ripple caused by filtering as is seen in single-carrier modulation does not occur, and thus the PAPR of the primary modulation depends only on a data mapping method. In the IEEE 802.11a standard, the mapping is performed so that <Ak>2 for all the sub-carriers equals to 1. Symbol points and their normalization coefficients are shown below.
BPSK: ±1
QPSK: (±1±j)/sqrt(2)
16 QAM: ([−3, −1, 1, 3)+[−3, −1, 1, 3]j)/sqrt(10)
64 QAM: ([−7, −5, −3, −1, 1, 3, 5, 7]+[−7, −5, −3, −1, 1, 3, 5, 7]j)/sqrt(42)
Since normalization coefficients 1/sqrt(2), 1/sqrt(10), and 1/sqrt(42) are fixed so that the power equals to 1 as described above, the PAPR of the primary modulation is 0 dB. Hence, the PAPR of the OFDM modulation equals to10*log(52+52*51)/52=10*log(52)=17 dBwhere, sqrt(x) represents a square root of the value x.
However, since it is very unlikely that all the sub-carriers take the maximum values simultaneously in the practical modulation, an output back off of the high-frequency power amplifier required for satisfying modulation accuracy stated in the IEEE 802.11a standard is approximately 7 dB. Here, the back off is a ratio of the output power obtained from the linear amplifier compressing a linear response by 1 dB (P1 dB) to the average output power.
That is, in the OFDM modulation, the high-frequency power amplifier operates using the power of 20% of P1 dB (=7 dB back off) as the average power. The remaining 80% is wasted without contributing to the power amplification. When considering the class A amplifier among the linear amplifiers as an example, the power efficiency in its saturated operation is 50%, while it operates with the power efficiency of 20% thereof, i.e., 10%, for processing the OFDM modulated wave. As described above, since the OFDM modulated wave has the large PAPR, the linear amplifier operates with the large back off, causing a problem of low efficiency in the high-frequency power amplifier.
Next, effects of using the EER method will be described.
The EER method, being one of means to solve the foregoing problems, separates the modulating signal represented by complex vectors as described above into the phase component and the amplitude component by converting to polar coordinates. The amplitude of the separated phase component is the constant modulating signal, so that the PAPR of the modulated wave is 0 dB. The high-frequency power amplifier can therefore operate in a saturated mode. The output power of the high-frequency power amplifier in the saturated mode is, regardless of the operation class, proportional to the square of the power supply voltage (voltage given to the drain or the collector) for a constant load. That is, the output voltage is proportional to the power supply voltage.
When the proportionally multiplied voltage by the amplitude component of the modulating signal as the power supply voltage of the high-frequency power amplifier is applied, under the condition that the high-frequency power amplifier operates in the saturated mode, the modulated wave of the phase component input into the high-frequency power amplifier is multiplied by the amplitude component in an output section of the high-frequency power amplifier, restoring the linear modulating signal. Described above is a basic operation of the ERR method. In this regard, although the high-frequency power amplifier operates in other than the saturated mode, i.e., the power supply voltage and the output voltage respond non-linearly, the EER method is operable by correcting the distortion.
Since the high-frequency power amplifier involves only the saturation power, non-linearity of the input/output response does not cause a problem. That is, the operation classes, such as the class B, C, F, E or the like, for operating the high-frequency power amplifier in high efficiency can be applied. The class B is the operation class to operate the high-frequency power amplifier as a half-wave rectifier, flowing the current only through a half section of the input voltage amplitude. As a result of Fourier series transform, while a fundamental wave component of the class B operation has the same amplitude with that of the class A operation and thus the output power is also the same, a DC component is 2/π. Thus, the saturation efficiency reaches 78.5%. The class C is the operation class which cuts back an interval length that the current flows, where an on/off duty ratio of the current shifts from 50%, so that the amplitude of the fundamental wave component is reduced resulting in the decrease in the output power. Moreover, since the DC component and the fundamental wave component vary intricately against the duty ratio, the class C operation may be less efficient than the class B operation depending on the duty ratio. The class F or E is the switching mode operation class, which operates the high-frequency power amplifier as a switch. Since an output current waveform gets closer to a rectangular wave, the efficiency is increased and, furthermore, the fundamental wave component of the current standardized at the maximum DC current gets closer to 2/π. As a result, the output power will increases (2/π)/(1/2)=4/π times (approximately 1 dB) at the maximum, compared to that of the class B (fundamental wave component is 1/2).
As described above, the EER method enables to power amplify highly efficiently, the modulating signal having the large back off.
Patent Document 1: U.S. Pat. No. 6,377,784 B2 (page 4 of the drawings, FIG. 9)
Patent Document 2: U.S. Pat. No. 6,528,975 B2 (page 4 of the drawings, FIG. 4)
However, in the conventional EER method, when the broadband modulating signal is applied, the operational amplifier 1907 requires a gain bandwidth (Hereinbelow, frequency range where the gain is 1: referred to as GBW) to be broader. For example, a baseband signal of the modulating signal has the band of 100 MHz, the band required for its amplitude component becomes approximately 5 times thereof, so that the operational amplifier 1907 needs to amplify the band of 500 MHz. In the case that the operational amplifier 1907 has a second-order roll-off characteristic, a voltage gain attenuates at 12 dB/oct, so that it requires the GBW of 2 GHz to obtain the gain of 6 dB (double) in the band of 500 MHz. For the implementation, it thus requires the development of a device capable of high-frequency operation, i.e., a device obtaining the high gain at the high frequency, and the circuit technology to compensate a phase delay in an internal circuit at high speed. Moreover, for the broadband modulating signal, it requires the higher-speed sampling frequency of the D/A converter. For the foregoing modulating signal, for example, the required sampling frequency is at least 1 Giga sample per second (Gs/s: double oversampling), requiring the development of an MOS switch capable of being turned on/off at high speed and the high-speed bus technology. As described above, broadbandization of such as the operational amplifier 1907 or the D/A converter, not only increases the power consumption, but also requires a breakthrough of new technology.